Online advertising has evolved from the early non-contextual pay-per-impression model to the currently predominant contextual pay-per-click model. In non-contextual pay-per-impression model, ads are placed on publishers' websites with little or no regard to visitors' interest. Payment from advertisers to ad brokers and from ad brokers to publishers are typically tied to the number of times a particular ad is displayed to visitors, hence the name pay per impression.
This model has in large part been replaced by the contextual pay-per-click model so that payment of an ad is directly tied into its performance. Advertisers only pay for the clicks (or number of times an ad was clicked by users), not the impressions or number of times an ad was displayed. Thus, in contextual pay-per-click online advertising models, the success of an ad is measured by its click-thru rate, which is defined as the number of times the ad is clicked by any user over the number of times the ad was displayed. Consequently, the more targeted the content, the higher the click-thru rate, and the more revenue for ad brokers and website publishers.
Typically, ad brokers profile subscribers via cookies. Cookies from an ad broker track end user clicks on the ads provided by the ad broker. An ad broker has no visibility to websites which an end user visits. As a result, an ad broker's ability to profile an end user is limited.